A passive optical network (PON) is a network architecture employing fiber cables from a central office to local premises. It employs passive optical components to enable a single optical feeder fiber to serve multiple premises. A PON consists of a central office node, where the optical line terminal (OLT) equipment is located, one or more termination nodes at customer premises, called optical network terminations (ONT) or optical network units (ONU), and), and the infrastructure such as fiber, splitters, filters, etc. connecting the central office node to the termination nodes, called the optical distribution network (ODN). In a passive optical network a single optical fiber, feeder fiber, guides the light towards the remote node (RN) where it is delivered to the different drop sections by means of data splitters, arrayed waveguide gratings (AWGs), filters, or any other passive equipment. From the RN the light is guided towards the customer premises: ONT if the unit serves one single home, ONU if the unit serves multiple homes. On the uplink, the ONT/ONU sends user data back to the OLT using the same or a different wavelength.
The primary reason for the PON choice has been its cost effectiveness because of the efficient use of the fiber and because of that most equipment outside the central office can be passive equipment that does not consume power. Compared to active optical networks (AON) PON can lower both operational expenditure (OPEX) and capital expenditure (CAPEX). Different forms of PON including Broadband PON (BPON), Ethernet PON (EPON), Gigabit Ethernet PON (GEPON), and Gigabit PON (GPON) have been deployed in different markets. Even though PONs have relatively low OPEX compared to active solutions, there is still room for the operators to save significant amount of OPEX using effective preventive maintenance of the physical infrastructure.
In today's PON systems, the physical infrastructure is usually not entirely visible to a network management system (NMS). A physical failure cannot be detected before creating service outage in upper layers which may lead to loss in business for the operators. The aim of preventive maintenance is to detect any kind of deterioration in the network that can cause suspended services and to localize these faults.
Supervision of monitoring of PONs should provide continuous, remote, automatic, and cost effective supervision of the physical layer. It should provide rapid and accurate detection of performance degradation as well as service disruption. The testing should not affect normal data transmission (non-intrusive testing). It should distinguish between a failure in the end-users' own equipment and a failure in the operator's network. It should be interoperable with many network variants (bit rate, protocol etc.).
A common maintenance tool employed for monitoring or supervision of PONs is an optical time domain reflectometer and the technique used is called optical time domain reflectometry (OTDR). The OTDR injects a series of optical pulses into the fiber under test. Backscattered (Rayleigh) and back-reflected (Fresnel) light from points along the fiber is detected and analyzed. The magnitude of the backscattered signal is dependent on the Rayleigh backscattering coefficient, attenuation, fiber imperfections and splices, and optical power level in the fiber. The strength of the return pulses is measured, integrated as a function of time, and evaluated as a function of fiber length. The OTDR may be used to estimate the fiber's length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks, and to measure optical return loss. The optical dynamic range of an OTDR is limited by a combination of optical pulse output power, optical pulse width, input sensitivity, and signal integration time. Higher optical pulse output power, and better input sensitivity, combine directly to improve measuring range, and are usually fixed features of a particular instrument.
OTDR monitoring technique is commonly used in PON systems. The same can be outlined for the Raman assisted OTDR, commonly planned to gain higher resolution in system fault detection. Off-the-shelf OTDR market equipment uses a fixed wavelength in the U-band of the ITU-T grid but for future access technologies as WDM PON, tunable OTDR devices would be required. Tunable OTDR enables selection of a specific drop section to be monitored without the need of an optical switch at the RN. However, many operators have already invested into OTDR equipment in the central office and are not interested in making further investments to upgrade the OTDR equipment.
Monitoring should not influence regular data communication, i.e. it should be non-invasive. This is achievable by utilization of a dedicated optical bandwidth for the measuring function. Further, the technique should be sensitive to relatively low power fluctuations detectable in on-demand or periodic modes. Still further, it should not require any high initial investment. This mainly yields that no additional monitoring functionality on the customer premises side should be needed and PON monitoring functionality should be shared over a complete PON system or a group of PON systems.
Today's existing solutions for providing supervision or monitoring do only satisfy some of the above requirements. An overview of some existing solutions is given in the article K. Yuksel, V. Moeyaert, M. Wuilpart, and P. Mégret “Optical Layer Monitoring in Passive Optical Networks (PONs): A Review”, ICTON 2008, Tu.B1.1. Most of the solutions existing today significantly increase capital expenditures because they require either a customized OTDR device, which is expensive, wavelength specific components in the fiber links (drop section) towards the ONTs, which causes power budget reduction, advanced OLT transmitter upgrades, e.g. light path doubling. Still further, most of today's existing solutions to provide supervision or monitoring can only detect a fault in a fiber link which introduces significant loss of more than 5 dB, far above an expected threshold of 1 dB.